1. Field of the Invention
The present invention relates to a method for assembly and installation of an offshore wind turbine, where components such as a nacelle comprising a shaft and a hub, one or more tower sections and/or blades are manufactured, assembled at or transported to a harbor area, where said components for one or more wind turbines are loaded onto a transport and installation vessel and subsequently transported to an erection site.
2. Description of Related Art
It is well known that transporting a wind turbine in the megawatt class can be a challenge due to the weight and size of at least some of the components of such a wind turbine. Typically, a wind turbine is transported in a number of pieces to an erection site and then assembled at the site. Such pieces will typically be a number of tower sections, a nacelle including a hub, and then the blades. The parts will be installed piece by piece at the erection site using cranes that fulfil the specific demands.
As the wind turbines becomes larger and larger in capacity so does the tower sections, the nacelles, the blades and other parts of the wind turbine. A typically nacelle can easily have a weight of more than 200 metric tons and can easily have a diameter or height of up to eight meters or more. Hoisting such a nacelle to its final position on top of a 100 meter high tower demands optimum conditions. The same goes for installation of the wind turbine blades that these days can have a length of up to 75 meters and in the near future even longer.
No matter if installation of the blades is performed by hoisting the blades to the nacelle on top of the tower, or if the blades are installed at the nacelle before hoisting the complete unit to the top of the tower the wind speed needs to be taken into consideration.
Hoisting of such components needs to be performed very accurately in order to be able to bolt the parts together without damaging the interfaces on said parts and one very important factor when it comes to installing wind turbines is actually the wind. It lays in the nature of an erection site for a wind turbine that it is a windy area which, of course, is perfect after installation, but which very often causes problems during installation.
During installation of wind turbines, it is thus very common having to pause the work for hours, days, or even weeks until the wind situation is suitable and steady for the jobs to be carried out. Onshore, this is of course annoying and causing extra costs, but this is actually nothing compared to the costs caused when an offshore installation has to be put on hold until the wind has a suitably low speed and perhaps also until the impact of the waves is minimized. Having all of the needed equipment in position for erecting an offshore wind turbine, but having to wait for better conditions is a major expensive problem. It is thus very attractive to assemble as much of a wind turbine onshore, at a factory or at the harbor area, prior to transporting the partly assembled wind turbine to an erection site. In turn, transporting partly assembled wind turbine components offshore will induce further challenges, mainly due to the influence that high and heavy components can have on the stability of the vessel used, as the center of gravity will be located relatively high in relation to the water surface.
Further, there is still a need for hoisting the components into place and these hoists still need conditions that are more or less the opposite of what is common for the site—namely low or no wind!
From U.S. Patent Application Publication 2011/0220538 A1 and corresponding U.S. Pat. No. 8,528,735 B2, a solution for transporting a nacelle comprising a rotor hub having three wind turbine blades is known. Additional parts of the wind turbine, such as tower sections, need to be installed separately from another vessel. Here, the nacelle and the rotor are installed in a special transport frame, which allows the nacelle to be shifted from a first position where the main shaft is more or less horizontal to a second position where the main shaft is in a less than vertical position. After having arranged the nacelle in this position, the rotor, comprising three wind turbine blades, is installed. As the main shaft is in a less than vertical position, it is possible to position more nacelles on the deck of a transport vessel having the rotors overlapping each other. A rather large disadvantage of using this method is that the complete nacelle comprising the rotor hub and the wind turbine blades needs to be hoisted from the transport vessel as a single unit, after which the transport frame has to be removed either before swinging the complete arrangement approximately 90° to a height well above the deck of the transport vessel or well above the water surface. Such an operation is very demanding and requires very good weather conditions, which evidently will incur large costs.
European Patent Application EP 2 463 511 A1 discloses a solution for transporting rotors comprising mainly two out of three wind turbine blades. The rotors are not installed at a nacelle and are arranged at the deck of a vessel with a inclined center axis of the hub. As a result, it becomes possible to have the rotors overlapping each other, and thus, to carry more rotors on one vessel. Also, in this solution, the rotor needs to be hoisted a certain distance above the deck of the vessel and then to be swung approximately 90 degrees in order to arrange the rotor in a position where it can be installed at a nacelle. Such an installation needs to take place at a nacelle that already is installed on a tower, meaning that the installation takes place approximately 80 to 120 meters, or even more, above sea level, making the operation much more sensitive to, e.g., wind.
Until now, no known solutions address the above mentioned problems of partly assembling and transporting a modern two-bladed megawatt wind turbine to an erection sites on a vessel where the partly assembled parts are loaded onto the deck of said vessel without having a significantly high center of gravity.